1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an active matrix organic electroluminescent device including a driving element having a plurality of thin film transistors interconnected in parallel and a fabricating method thereof.
2. Discussion of the Related Art
In general, an organic electroluminescent device (ELD) emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Compared to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the exciton between the two states causes light to be emitted. Accordingly, the size and weight of the organic ELD can be reduced. The organic ELD has other excellent characteristics such as low power consumption, superior brightness, and fast response time. Because of these characteristics, the organic. ELD is regarded as a promising display for next-generation consumer electronic applications such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is a simple process with a few processing steps, it is much cheaper to produce an organic ELD than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabricating process, the passive matrix organic ELD requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic. ELDs are highly efficient and can produce a high-quality image for a large display with a relatively low power.
FIG. 1 is a schematic cross-sectional view of an organic ELD according to a related art. In FIG. 1, an array element 14 including a thin film transistor (TFT) “T” is formed on a first substrate 12. A first electrode 16, an organic electroluminescent layer 18, and a second electrode 20 are formed over the array element 14. The organic electroluminescent layer 18 may separately display red, green, and blue colors for each pixel region. A second substrate 28 faces the first substrate 12 and is spaced apart from the first substrate 12.
The first and the second substrates 12 and 28 are attached to each other with a sealant 26. The organic ELD is encapsulated by attaching the first substrate 12 to the second substrate 28. The second substrate 28 includes a moisture absorbent material 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic electroluminescent layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent material 22 and the filled moisture absorbent material is fixed by a holding element 25.
FIG. 2 is an equivalent circuit diagram of the organic electroluminescent device according to the related art. In FIG. 2, a gate line 36 crosses a data line 49, and a switching element “TS” at a crossing of the gate line 36 and the data line 49 is connected to the gate line 36 and the data line 49. A driving element “TD”electrically connects the switching element “TS” to an organic electroluminescent diode “DEL.” A storage capacitor “CST” is formed between a driving gate electrode 34 and a driving source electrode 52 of the driving element “TD” as the driving element “TD” is a positive type transistor. The organic electroluminescent diode “DEL” is connected to a power line 62, and the driving drain electrode may be connected to an anode of the organic electroluminescent diode “DEL.”
When a scan signal of the gate line 36 is applied to a switching gate electrode 32 of the switching element “TS” an image signal of the data line 49 is applied to the driving gate electrode 34 of the driving element “TD” through the switching element “TS.” The current density of the driving element “TD” is modulated by the image signal applied to the driving gate electrode 34. As a result, the organic electroluminescent diode “DEL” can display images with gray scale levels. Moreover, because the image signal stored in the storage capacitor “CST” is applied to the driving gate electrode 34, the current density flowing into the organic electroluminescent diode “DEL” is uniformly maintained until the next image signal is applied, even when the switching element “TS” is turned off. The switching element “TS” and the driving element “TD” can be a polycrystalline silicon TFT or an amorphous silicon TFT. The process of fabricating an amorphous silicon TFT is simpler than the process for a polycrystalline silicon TFT.
FIG. 3 is a schematic cross-sectional view illustrating a switching element and a driving element including an amorphous TFT for one pixel region of an organic electroluminescent device according to the related art. In FIG. 3, a gate line 36 is formed on a substrate 30 in a first direction, a data line 49 crosses the gate line 36 in a second direction, and a power line 62 is arranged in parallel to the data line 49 and crosses the gate line 36. A pixel region “P” is defined by the gate, data and power lines 36, 49 and 62. A switching element “TS” adjacent to the pixel region “P” is connected to the gate and data lines 36 and 49. A driving element “TD” is connected to the switching element “TS.” In addition, the switching element “TS” includes switching gate electrode 32, switching active layer 56a, switching source electrode and drain electrode 48 and 50. The driving element “TD” includes a driving gate electrode 34, a driving active layer 58a, a driving source electrode 52 and a driving drain electrode 54. Specifically, the driving gate electrode 34 is connected to the switching drain electrode 50, the driving source electrode 52 is connected to the power line 62, and the driving drain electrode 54 is connected to the organic electroluminescent diode “DEL”. (of FIG. 2). The switching active layer 56a and the driving active layer 58a may be formed of amorphous silicon.
The amorphous silicon driving TFT should have a large width to length ratio (W/L ratio) in order to drive the organic electroluminescent diode “DEL” (of FIG. 2). In this case, a size of the driving element “TD” is much larger than a size of the switching element “Ts.”
FIGS. 4A and 4B are schematic cross-sectional views taken along the lines “IVa—IVa” and “IVb—IVb” in FIG. 3, respectively.
In FIGS. 4A and 4B, a switching gate electrode 32 and a driving gate electrode 34 connected to the switching gate electrode 32 are formed on a substrate 30. Although not shown in FIGS. 4A and 4B, a gate line is formed on the substrate 30 in a first direction and connected to the switching gate electrode 32. A gate-insulating layer 38 is formed over the substrate 30 including the switching gate electrode 32 and the driving gate electrode 34. A switching semi-conductor layer 56, and a driving semi-conductor layer 58 are formed over the switching gate electrode 32 and the driving gate electrode 34, respectively. The switching semi-conductor layer 56 has an island isolated shape and includes a switching active layer 56a and a switching ohmic contact layer 56b. In addition, the driving semi-conductor layer 58 also has an island isolated shape and includes a driving active layer 58a and a driving ohmic contact layer 58b. 
Switching source and drain electrodes 48 and 50, and driving source and drain electrodes 52 and 54 are formed on the switching semi-conductor layer 56 and driving semi-conductor layer 58, respectively. Specifically, the switching source and drain electrodes 48 and 50, and the driving source and drain electrodes 52 and 54 contact the switching ohmic contact layer 56b and the driving ohmic contact layer 58b, respectively. In addition, the driving gate electrode 34 is connected to the switching drain electrode 50. A first passivation layer 60 is formed over the substrate 30 including the switching source and drain electrodes 48 and 50, and the driving source and drain electrodes 52 and 54. A power line 62 is formed on the first passivation layer 60 and connected to the driving source electrode 52. A second passivation layer 64 is formed over the substrate 30 including the power line 62, and a first electrode 66 is formed on the second passivation layer 64 in the pixel region “P” and is connected to the driving drain electrode 54.
The driving active layer 58a of the organic electroluminescent device according to the related art has a large value of width to length ratio (W/L ratio) in order to provide the organic electroluminescent diode with enough currents, which affects the aperture ratio adversely. Moreover, because current stress increases as the current density increases, thermalization of the driving TFT may occur. Furthermore, because direct current (DC) biases are constantly applied to the driving element, operational properties of the driving element varies. Accordingly, an active matrix organic electroluminescent device having such an amorphous silicon TFT has a poor image quality such as residual image, and the poor operational properties of the driving element lead to point defects in the active matrix organic ELD.
Meanwhile, when an array element layer of TFTs and an organic EL diode are formed together on one substrate, the production yield of an organic ELD is determined by a multiplication of the yield of the array element and the yield of the organic EL diode. Since the yield of the organic EL diode is relatively low, the production yield of an organic ELD is limited by the yield of the organic EL diode. For example, even when TFTs are well fabricated, an organic. ELD can be determined to be defective due to a defect of an organic emission layer of a thin film of about 1000 Å thickness. This results in loss of materials and high production cost.
Organic ELDs are classified into a bottom emission type and a top emission type according to transparency of the first and second electrodes of the organic EL diode. The bottom emission type ELDs have advantages such as high image stability and various fabrication processes due to encapsulation. However, the bottom emission type organic ELDs are not adequate for devices requiring a high resolution due to the limitations in aperture ratio. On the other hand, since the top emission type organic. ELDs emit light in a direction upward of the substrate, light emits without influence of the array element layer that is positioned under the organic EL layer. Accordingly, the overall design of the array layer including TFTs may be simplified. In addition, the aperture ratio can be increased, thereby increasing operational life span of the organic ELDs. However, because the top emission type organic. ELDs have a cathode commonly formed over the organic EL layer, material selection is limited so that light transmission efficiency is lowered. When a thin film type passivation layer is formed to prevent the reduction of light transmittance, the thin film type passivation layer may fail to prevent infiltration of exterior air into the device.